watt?
December 29, 2009 7:47 PM   Subscribe

If I have a stereo amplifier that puts out 200 watts per channel, with two speakers per channel, how many watts per speaker?
posted by 31d1 to Science & Nature (26 answers total) 1 user marked this as a favorite
 
100, I assume — how are the two speakers per channel attached?
posted by hattifattener at 7:50 PM on December 29, 2009


Best answer: 100W. Voltage and resistance have funny rules to remember that depend on how you wire the speakers (series vs parallel). But power is always additive.

I'd go a little more because speakers don't always perform the best at the limits of their rating.
posted by sbutler at 7:51 PM on December 29, 2009 [1 favorite]


Response by poster: Thanks sbutler.

I suppose that in terms of voltage and resistance an amp with a left and right per channel would be in series and I'd have to be plugging a speaker into a speaker into a left or right to get or worry about parallel.
posted by 31d1 at 8:30 PM on December 29, 2009


um... no. Left and right are the channels. If you connect two speakers to a given channel, then unless you do something unusual, those speakers will end up in parallel.

Also: amplifiers are usually rated at a certain output power into a certain nominal impedance (e.g. 200 watts per channel into 8 ohms). Often they will be rated rather higher into a 4 ohm load than into an 8 ohm load; if you have an amp rated 200W/channel into 8 ohms, it might also be rated something like 320W/channel into 4 ohms. You really have to check the specs to find out.

Most domestic stereo speakers are nominal 8 ohm loads. Car stereo speakers tend to be 4 ohms.

If you put a pair of 8 ohm speakers in parallel on each channel, the channels will see 4 ohm loads, so the total power available per channel will be whatever the specs tell you is right for 4 ohms; and it's that figure, not 200W, that will be divided equally between the speakers.

There are not many amps that deal well with 2 ohm loads, so putting a pair of 4 ohm speakers in parallel is probably not a great idea.
posted by flabdablet at 8:46 PM on December 29, 2009


Best answer: sbutler is right in the abstract but in practice your amp will probably be able to deliver different power into different loads, which could change things.

Take one side of your amp, say it is rated at 200W into 8 ohms. You have two 8 ohm speakers. You wire these in parallel. One of four things will happen:
- Your amp will deliver a clean ~400W into the 4 ohm load that its seeing now [unlikely, unless your amp is expensive]
- Your amp will deliver 400W but it'll be all distorted at higher levels/large transients because its power supply is incapable of dishing up that much power [most likely]
- Your amp will see a 4 ohm load, but will have some sort of regulation circuitry to keep it from trying to output more than 200 watts for more than a very brief transient [medium likely]
- Your amp will melt, because its just not designed to push this low-impedance of a load at this level. There is a difference between an amp and an arc welder. If you short the terminals of nearly any reasonable amp together, it will burn up. The specific impedance and level at which it burns up is an engineering tradeoff. [not that likely]

If you wire your 8 ohm speakers in series, your amp sees a 16 ohm load, which it may or may not still be able to deliver 200W into.

The point is if your amp is actually putting out 200W, your speakers will indeed see 100W, but its quite likely that your amp will deliver more power into the lower impedance load, and you want speakers that are rated for higher power handling if you plan on running this at full output, unless you want your voice coils to melt.

I guess what I'm getting at here is while power is additive, actual amps will be capable of delivering different power levels into different loads, and so the total power your speakers are being called upon to handle may change.
posted by jeb at 8:48 PM on December 29, 2009


I suppose I should expand on my earlier question. The power the amp puts out will be the sum of the power delivered to the speakers (neglecting various things that one can generally neglect). But there's no guarantee that it'll be evenly distributed among those speakers.
posted by hattifattener at 8:52 PM on December 29, 2009


Impedance is confusing. It's expressed in ohms, just like resistance, but impedance is not the same as resistance. Properly it should be represented as a curve, not as a single number.

The single number is kind of an average. But unless the two speakers are nearly identical (i.e. same brand, come out of the same factory, at about the same time) their curves may not be the same even if they're both nominally rated "8 ohms".

If the speakers are different brands or models, then there isn't any simple answer to your question. It depends not only on the impedance curves (nominal resistance per frequency) of the two speakers, but also on the characteristics of the music you're playing, its amplitude-per-frequency curve, which presumably is constantly changing.

And that's ignoring the question of whether the amplifier has head room above its nominal 200 watt rating, and how it behaves when overdriving.
posted by Chocolate Pickle at 8:59 PM on December 29, 2009 [1 favorite]


Best answer: Let me try to expand on that a bit. Suppose one of the speakers has better treble response, because it has a lower impedance at high frequencies. The other has better base response, and lower impedance at low frequencies.

If the music you're playing is mostly treble, then the speaker with lower impedance at high frequencies will draw more power. If the music suddenly changes to predominantly bass, then suddenly the other speaker will draw more power.

And depending on what the specific impedances are at frequency, the combined impedance won't necessarily be 4 ohms (assuming the speakers are in parallel). It could be higher or lower as a function of the specific impedances of the two speakers at the particular frequency which is dominating the music being played at that particular instant.
posted by Chocolate Pickle at 9:02 PM on December 29, 2009


Response by poster: OK I need to simplify this a bit. The amp has a left and right per channel. Four separate outs. Two channels. You are saying this is likely to be parallel, not series?
posted by 31d1 at 9:09 PM on December 29, 2009


Response by poster: Because all the stuff you guys are saying makes some sense, but knowing whether two 8 ohm speakers are behaving more like 4 or 16 is a bit of a big deal. the manual for this thing talks all about impedance and warns against less than 4 ohms, it just doesn't say simple things like whether if I plug in 4 speakers I am increasing or decreasing the load.
posted by 31d1 at 9:28 PM on December 29, 2009


If the amp has four separate outs, and it's a good amp, it's entirely possible that each of the four has its own final power stage. Which would mean they are neither serial nor parallel; they'd be completely independent.
posted by Chocolate Pickle at 9:33 PM on December 29, 2009


Response by poster: Wouldn't four independent outs be stated as four channels, not two?

To avoid making me making me more confused, I will note at this point that I am thinking about this amp in particular (although I made up numbers for the purposes of the original question regarding wattage). And it seems like I'm supposed to know how different combinations of speakers should behave with it, although there is no simple indication of how the thing is wired.
posted by 31d1 at 9:48 PM on December 29, 2009


They wouldn't be independent.
posted by Chocolate Pickle at 9:52 PM on December 29, 2009


The amp has a left and right per channel. Four separate outs. Two channels.

Unless it's a really rare thing, you're probably misunderstanding this.

Even if your amp has four speaker connectors on the back panel, by far the most common arrangement is not to have four output channels driving those, but two: Left and Right. The connectors will be labelled something like Speakers 1 (left + right) and Speakers 2 (left + right) and you will have corresponding front-panel switches allowing you to connect Speakers 1, Speakers 2 or both to the amp's outputs. If you do connect both, Speaker 1 Left and Speaker 2 Left get connected in parallel to the amp's left channel output, and Speaker 1 Right and Speaker 2 Right get connected in parallel to the amp's right channel output.

So if you have one 8-ohm speaker connected to each back-panel connector, your amp will see a pair (left+right) of 8-ohm loads any time you have either Speakers 1 or Speakers 2 selected on the front-panel switch, or a pair (left+right) of 4-ohm loads if you have both Speakers 1 and Speakers 2 selected.
posted by flabdablet at 10:09 PM on December 29, 2009


Going a bit *ahem* parallel: what speakers are you thinking of powering with that amp? What kind of gigs do you want to use your system for?
posted by Monday, stony Monday at 10:22 PM on December 29, 2009


The pictures on the site you linked are too coarse to read the labels on front and back panels. Are better pictures available?
posted by flabdablet at 10:35 PM on December 29, 2009


(Also: that website is full of spelling mistakes. The product might be fine, but personally, unprofessional websites [even with a white background, nacht] make me a little wary)
posted by Monday, stony Monday at 10:53 PM on December 29, 2009


Just to throw another spanner into the understanding works, that amp also has a Bridged mode to give you lots and lots of power (up to 1000W claimed). Bridged mode allows you to combine the power of two channels to drive a single speaker. Effectively, Bridged mode gives you a double-power, single channel amplifier. I suspect that this particular unit would allow you to apply that single channel to at least two speakers, but I'd have to see the product manual to be sure of that.

Bridged mode will require double the speaker impedance of non-bridged mode, for safe operation; maximum power for that amp in bridged mode is specified at 8 ohms. So you wouldn't run bridged mode into a 4 ohm speaker or (equivalently) a parallel pair of 8 ohm speakers.

The other confusing thing about those specs is their use of both EIAJ and continuous specifications. The EIAJ specs (which are higher) are fairly meaningless - they say what the amp can do in short bursts that may or may not correspond to anything that's actually going to be played through them. When you're comparing amps, the RMS or continuous power rating is the one that's worth paying attention to.

Also, if the reason you're asking this question is to figure out what power rating you'd need on speakers to be driven by this amp: you'll only ever get a really crude answer to that. It's quite possible to blow up a set of speakers rated at 100W (continuous) with an amp rated at 50W (don't ask me how I know that for sure - the memories are painful) - all you need to do is supply a signal that's either got a fairly narrow frequency spectrum, so that all the amp's power ends up being soaked up by the midrange or tweeter and none by the woofer. Driving a power amp into clipping will also burn voice coils, even in quite conservatively rated speakers. The general rule is that if you're hearing more distortion as you turn the amp up, turn it down until that goes away!
posted by flabdablet at 11:19 PM on December 29, 2009


Response by poster: I'm actually just gonna be using it to power some fairly crappy but loud ~200 watt home stereo speakers that I use for monitors and extra-PA-in-the-corner when I spin records at the bar.

I haven't found better pictures online, the labeling on the back is just saying how the Speakon cable connections work, the manual is available here. The top two things on the back are XLR inputs, the bottom 4 are outputs.
posted by 31d1 at 11:28 PM on December 29, 2009


Best answer: OK. Closest I can get by carefully interpreting the sacred scrolls is that this amp is intended mainly for use with 8 ohm speakers.

In non-bridged mode, all four back-panel output connectors are active. The top left and bottom left speaker connectors will be wired in parallel, as will the top right and bottom right connectors. If you connect a pair of 8 ohm speakers to the top connectors or bottom connectors only, the amp will see an 8 ohm load on each channel and be capable of delivering 270W max to each speaker. If you connect a pair of 4 ohm speakers that way, the amp will see a 4 ohm load on each channel, and will be capable of delivering 500W max to each speaker. If you connect 8 ohm speakers to all four connectors, the amp will see a 4 ohm load on each channel, and be capable of delivering 500W to that load, which will split as 250W per speaker if the speakers are identical.

In bridged mode, the amp ignores its right input and only the top left output connector is active. If you connect an 8 ohm speaker to that connector, the amp will be able to supply it with 1000W max. If you connect a series pair of 8 ohm speakers to that connector, the bridged output will see a 16 ohm load and be capable of supplying that with 520W max total; if the two speakers are identical, they will each see 260W of that. If you connect a 4 ohm speaker, or a parallel pair of 8 ohm speakers, the amp will operate outside its specifications and go into thermal shutdown when pushed.
posted by flabdablet at 11:43 PM on December 29, 2009


The distilled version: except in bridged mode, this amp can deliver roughly 250W max to an 8 ohm speaker. So for your ~200W speakers, flip its limiter to 300W and you should hear bad distortion before they're in danger of burning out.
posted by flabdablet at 11:49 PM on December 29, 2009


that website is full of spelling mistakes

As is the manual; fairly typical for commodity Chinese equipment.
posted by flabdablet at 11:50 PM on December 29, 2009


I see it's $200 on Musician's Friend; there are some "factory restock" QSC GX3s on there for $250. You'd have to call to confirm you get it, but QSC amps normally come with a 3-year warranty, extendable to 6 for free in the US. Their customer service is good, and their manuals written by literate people. The GX3 would give you 2 channels of 300 watts into 8 ohms or 425 watts into 4 ohms. The GX3 cannot be bridged however, and at 26 pounds it may or may not be heavier than the Vestax.
posted by Monday, stony Monday at 12:03 AM on December 30, 2009


Electrical engineer here, with some sound reinforcement work on the side. The correct answer is: amplifier power specifications are imprecise and unimportant. The difference between 100W and 200W is often nothing more than honesty. But this is ok because any discrepancies in output power are completely overshadowed by things like speaker efficiency, room acoustics, and the logarithmic nature of perceived volume.

Of course, it's possible to have too little power for a given room/event/situation, but you're not going to discover that beforehand by doing some calculations and checking the amp specs. Instead, you make a guess and try to err on the side of too much (nobody ever complains about having too much).
posted by ryanrs at 6:39 AM on December 30, 2009


you should hear bad distortion before they're in danger of burning out.

Somewhat counterintuitive, but your tweeters will be damaged sooner by an underpowered amp than by an overpowered one. Once the amp starts distorting, the discontinuous waveform is worse for them than a louder but undistorted signal.
posted by StickyCarpet at 3:58 PM on December 30, 2009


As a general rule its best to match a higher wattage amplifier with a lower wattage speaker.

If you drive a speaker too hard it will usually distort in a pretty obvious way (the coil will bottom out) giving you a chance to dial back the volume before the coil blows.

Conversely if you drive an amplifier to distortion it will start feeding distorted square waves to your speakers - this is less audible, (the speaker is still within its nominal power limits) - but this distortion will have the effect of feeding very high (inaudible) frequencies to the tweeter which will overheat and blow out.

Also remember that power is logarithmic, there is far less difference between 50W and 200W than most people think.
posted by Lanark at 4:01 PM on December 30, 2009


« Older Huh? Wha?   |   fiscal fashion har har har pls shoot me Newer »
This thread is closed to new comments.